1. Field of the Invention
This invention relates to a transparent thin film transistor and a display device having the same and, for example, it relates to a thin film transistor capable of coping with higher image quality and higher definition of a display device, as well as a display device using the thin film transistor described above.
2. Description of the Related Art
Higher image quality and higher definition for display devices have been developed rapidly in recent years and there has been a demand for reduction of the size of pixels. Usually in display devices using liquid crystals, a system of using thin film transistors (TFT) and driving them on every pixel has been adopted. For the operation, TFT using amorphous silicon is usually used, and liquid crystals driven by transparent pixel electrodes connected with transistors can be seen through back light. In this case, since amorphous silicon forming the transistor is photo-conductive, it is necessary to shield light so that the light from the back light for the display of liquid crystals is not incident to amorphous silicon for accurate operation only by the voltage, as well as light shielding is necessary for preventing erroneous operation also in a case of utilizing light incidence from the outside as in a reflection type since amorphous silicon is sensitive to visible light, which, however, results in a problem of complicating the steps. Further, since amorphous silicon has absorption for visible light, it is impossible to adapt a structure in which light is transmitted through the TFT portion.
The size of the pixel is determined depending on the size of a pixel electrode and the size of TFT to drive the same and it is necessary to reduce the size of the pixel in order to attain higher image quality and higher definition. However, when the size of the pixel is reduced, an area ratio (open ratio) between a pixel-forming TFT portion not transmitting light and a pixel electrode capable of transmitting light and driving liquid crystals is decreased to result in a problem of lowering the light transmittance and darkening the display, and also increasing electric power consumption.
Further, since the pixel itself depends on the area of the pixel electrode excluding the opaque TFT, it results in a problem of reducing the size of the pixel.
In view of the problems described above, a proposal has been made for the possibility of a transparent TFT using amorphous GaN and microcrystalline GaN (S. Kobayashi, et al. Applied Surface Science, 113/114 (1997) 480 to 484). In this report, a structure of using n-type crystalline Si as a substrate and gate electrode, and SiO2 as an insulative layer, and disposing a source-drain electrodes of an Al vacuum deposition film on microcrystalline GaN is used. In the device structure, while the semiconductor layer is transparent, TFT itself is not transparent. Further, although a structure of using a glass substrate has also been proposed (J. Cryst, Growth 189/190 (1998) 749 to 752), since it uses the source-drain electrodes of an Al film and the Al gate electrode, and it cannot be used for a transparent device.
This invention at intends to overcome the foregoing problems in the related art and provides a thin film transistor which is transparent and capable of transmitting light.
This invention provides a thin film transistor capable of obtaining a bright display device which has fewer light untransmissive portions in the pixel unit, capable of reducing the size of the pixel unit to the limit size of the transistor, manufactured by simple manufacturing steps, bright and with less power consumption, as well as a display device having such a thin film transistor.
The thin film transistor of this invention includes, on a transparent substrate, a transparent semiconductor layer containing nitrogen, hydrogen and one or more elements selected from Al, Ga and In, a transparent source electrode and a transparent drain electrode at least partially in contact with the transparent semiconductor layer, and a transparent gate electrode.
The thin film transistor according to this invention has, as the semiconductor layer, a semiconductor layer containing nitrogen, hydrogen and one or more elements selected from Al, Ga and In, and, since the substrate, the semiconductor layer and each of the electrodes are transparent, the transistor itself can transmit light. Particularly, in a case of utilizing the transistor for the display device, the pixel unit has fewer light untransmissive portions, the size of the pixel unit can be reduced to the limit size of the transistor and it can cope with higher image quality and higher definition of the display device. Further, since the semiconductor layer described above has no absorption for visible light and there is no requirement of separately providing a shielding member, manufacturing steps are simple. Since the transistor itself transmits light and this can eliminate the problem of the area ratio between the TFT portion and the pixel electrode (opening ratio), bright and less-power-consuming display device can be obtained. Further, the degree of freedom for the positioning of pixel electrodes is also enhanced.